The present invention relates to a robotic lawn mower, comprising an electric motor, preferably supplied by a rechargeable battery, and being provided with a cutting head having an output shaft which is connected to a cutting disc located perpendicularly to the shaft, said cutting disc comprising at its periphery at least one blade.
Lawnmowers with rechareable batteries are designed in order to run with a minimum power. The cutting device must be adapted to this primary need. It is more particularly the case for mowers which are entirely robotic, with unattended operation, such as the one described in the international patent application PCT/BE91/00068.
Traditional cutting systems perform the cutting operation under a cover, the cut grass being carried along by the rotating system. The ensuing result is that an important part of the energy is wasted during the cutting operation in order to transfer a kinetic energy to the cut grass which is dragged away by the cutting blade.
The cutting head usually used in robotic machines with a rechargeable battery is most often made of a lower plate, essentially circular, assemled to an upper plate by attaching flanges in such a way that a narrow space is left between the two plates.
A cutting disc is located between such plates and fixedly attached to a vertically extending output shaft from an electric motor. The cutting disc comprises preferably at least two retractable blades on its periphery, each of them pivoting around a shaft located on said periphery and capable of returning between said plates.
When the cutting disc starts to rotate, driven by the electric motor, the blades extend due to the centrifugal force and protrude out from both fixed plates in order to perform the grass cutting operation.
The rotating part in contact with the grass is then dramatically reduced and this arrangement decreases substantially the friction on the surface to be mowed, the carrying along or drawing away of the grass by the rotating system and the aerodynamic turbulence.
The use of such lower static plate entails however some problems.
In spite of the centrifugal effect due to the rotating disc, grass debris may on the long run and more particularly in wet weather, adhere to the internal face of the lower plate resulting in a braking effect with concomitant increase of energy consumption.
A low energy consumption is a vital criterion for an efficient performance of an electric lawnmower, more particularly a robotic mower with rechargeable battery.
In particular, for such an autonomous lawnmower with a battery which is rechargeable by induction at a fixed station, it is important that the search for this fixed station according to a predetermined algorithm (such as disclosed in patent application PCT/BE95/00013) be performed with a minimum energy consumption. For this purpose, the rotating cutting disc is disabled and will not operate during the searching step.
The lower plate acts then as a crumb scoop and scrapes the surface to be mowed with ensuing penetration of earth, twigs or dead leave debris between the plates. This may create a problem when the cutting disc has to be restarted.